CHAPTER X. 



THE SENSE AREAS AND THE ASSOCIATION AREAS OF 

 THE CORTEX. 



The delimitation of the sensory areas in the cortex is a matter 

 of very considerable difficulty, owing partly to the fact that the 

 determination of the presence or absence of certain states of con- 

 sciousness in the animal or person under observation cannot be 

 made except by indirect means, and partly no doubt to the fact 

 that the organization of the sensory mechanism in the brain is 

 more complex and diffuse than in the case of the motor apparatus. 

 Moreover, the distinction between what we may call simple sensa- 

 tions and the more complex psychical representations and judg- 

 ments of which these sensations form a necessary constituent can- 

 not be made clearly, even by the individual in whom the reactions 

 occur. We recognize in ourselves different stages in the degree of 

 consciousness aroused by sensory reactions. Our visual and 

 auditory sensations are clearly differentiated; but many of the 

 lower senses escape recognition in the individual himself, since the 

 state of consciousness accompanying them is of a lower order. 

 Our muscular sensations, for instance, are so indefinite as to be 

 practically subconscious. They are most important to us in every 

 act of our lives, yet the uninformed person is unconscious of the 

 existence of such a sensation, and if deprived of it would recognize 

 the defect only in the consequent loss of control of the voluntary 

 muscular movements. In the attempts to determine in what part 

 of the brain the various sensations are mediated every possible 

 method of inquiry has been used : the anatomical course of the 

 sensory paths, physiological experiments of stimulation and ablation, 

 and observations upon individuals with pathological or traumatic 

 lesions in the brain. In the long run, the study of neuropatholog- 

 ical cases in man must give us the last word, because in such cases 

 the estimate of the sensory defect can be made with most accuracy 

 and because in man the specialization of the psychical functions 

 has reached its highest development. The results that have been 

 obtained are perhaps the most definite in the case of the higher 

 senses, vision and hearing, since defects in these senses are recog- 

 nized most clearly, and the anatomical mechanisms involved have 

 proved to be more accessible to investigation. 



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